I have a couple of Robotron 1715 in my collection and both are dead (1715 and 1715M). For 1715 I managed to fix PSU (http://asknotes.com/2021/09/01/fixing-robotron-1715-psu/), for another PSU is still dead. But having one working PSU I decided to hold on with the other PSU repeat and get to the PC repair itself.
The main symptom is, after powering on, floppy drives are not seeking.
This is the main board of 1715 removed from the case with main components marked:
This is the mainboard layout of 1715:
This is schematics of 1715:
Testing the main board
First I decided to perform main board test using RAM TEST utility ROM:
I have burned RAMTEST_rom_v01.BIN to 28C16.
Test was showing no output via V.24 output to PC. So I tested CPU with oscilloscope and there was no clock signal on CPU pin 6.
I have checked schematics and this part is responsible to generate clock signal:
A33 was showing no output. A8 and Q2 output was correct, so most probably A33 has failed there.
For the sake of the test, I fired up the signal generator with 2.458Mhz square wave signal and injected it to pin 9 of A33.
Immediately pin 6 of CPU showed a clock signal, and other pins indicated that CPU started to work. V.24 output to PC showed ram test is running.
From the test it is seen that one RAM ic has failed. It is fourth bank, third ic from the bottom. I have tried piggybacking ram ic but it did not help. So, replaced failed ic with socket and new one, rerun ram test and now test has passed.
Later I have replaced Robotron 1715 A33 from clock generator circuit.
Then I installed the original ROM IC, and assembled the PC. After power up it started consecutively seek the first and second floppy drives and that means that PC is working.